1. Field of the Invention
This invention relates, in general, to camera imaging apparatus and, more specifically, to data processing apparatus for camera imaging devices.
2. Description of the Prior Art
In the operation of an image tube based camera, the sweep beam of the camera makes a plurality of vertically spaced, substantially horizontal sweeps across the image tube target of the camera. Typically, for example, the beam makes from 2621/2 to over 1000 horizontal sweeps across the image tube target before returning to the top of the target and repeating the scanning operation. A current is generated when the beam strikes the target which is amplified to generate a raw camera analog video signal data suitable for subsequent processing or display.
The sweep of the camera beam is divided into a scan period, i.e., a rapid left to right movement of the beam across the face of the image tube during which the beam also moves vertically downward at a slower rate, and a blank or retrace period during which the beam moves from right to left. During the retrace periods, the raw analog video output of the camera is normally blanked and frequently synchronization pulses (sync tips) are added to the video signal to indicate the start of the retrace period to external devices, such as monitors or data processors.
Due to the inherent design of a camera, a voltage or charge builds up on the image tube target during the scan and retrace periods which causes the raw analog output voltage to be offset or different from the original detected voltage by a d.c. error voltage. This introduces error into the raw camera analog video signal. Such errors result from leakage camera tube current over varying temperature ranges as well as a.c. coupling between various stages of the camera circuitry. Another source of the d.c. error results from the analog d.c. restore circuit which is commonly used to make the voltage at the camera output zero at the end of each scan. However, the d.c. restore circuit includes an R-C network which inherently introduces a time constant in reducing camera output voltage to zero. Thus, the camera output voltage never actually reaches zero but is brought to approximately zero over a certain time period. During this time period, the camera output remains at an error voltage and is not true zero.
In order to overcome these problems, it would be desirable to provide a camera data processing apparatus which overcomes the problems of previously devised cameras with regard to the excess charge build-up on the camera image tube target during scan and retrace periods as well as d.c. restoration errors. It would also be desirable to provide a camera processing apparatus which adjusts the raw camera analog video data to compensate for these errors on a scan line by scan line basis.